For example, in a conventional image encoding device described in the following nonpatent reference 1, an inputted color image is partitioned into largest coding blocks each having a predetermined size, and each largest coding block is further partitioned hierarchically into smaller coding blocks.
Each coding block is further partitioned into smaller prediction blocks, and an intra-screen prediction and a motion-compensated prediction are performed on each of the prediction blocks to generate a prediction error.
Further, the prediction error is divided hierarchically into transformation blocks within each coding block, and each of the transform coefficients is entropy-encoded, thereby achieving a high compression ratio.
In a conventional image encoding device, in order to implement high-efficiency encoding using a temporal correlation, a block having a high correlation with a coding target block is searched for from an already-encoded picture according to, for example, an encoding configuration as shown in FIG. 30(a), and encoding using a motion-compensated prediction which provides the block which is searched for as a predicted value is carried out. In general, a picture used for the motion-compensated prediction at that time is referred to as an inter picture, a picture which is a destination of the search (reference) in the motion-compensated prediction is referred to as a reference picture, and a picture which is predicted from only already-encoded pixels in a coding target picture without using the motion compensation is referred to as an intra picture. Particularly, it is known that a bidirectional prediction which is motion compensation which refers to both a past picture and a future picture in the display order as shown in FIG. 30(b) can implement a high-accuracy prediction. However, providing a reference relation between pictures by using motion compensation, as shown in these examples, results in dependence occurring in the decoding of each picture, and, as a result, decoding of an encoded bitstream (encoded stream) cannot be carried out from a picture existing at some midpoint of the bitstream and a halfway playback of a video image cannot be carried out.
Therefore, when using the encoding configuration using a motion-compensated prediction as shown in FIG. 30 or the like, there is a case of preparing a random access point showing a halfway decoding start point in order to make it possible to normally perform a playback even if the encoded bitstream is decoded from some midpoint of the bitstream. For example, an example of setting a randomly-accessible picture (an Intra Random Access Point (IRAP) picture described in nonpatent reference 1) is shown in FIG. 31. While an inter picture is allowed to refer to a picture preceding to an intra picture in time, an inter picture subsequent to an IRAP picture in the decoding order (encoding order) and also in the display order cannot refer to a picture preceding to the IRAP picture in time across the IRAP picture. More specifically, a limitation of the reference destination of the motion-compensated prediction is imposed on an inter picture subsequent to an IRAP picture in the decoding order (encoding order) and also in the display order. In this case, also when starting the decoding from some midpoint of the encoded bitstream, by starting the decoding from an IRAP picture, pictures whose positions in the display order are subsequent to that of the IRAP picture can be always decoded normally, and a halfway playback of the encoded sequence can be implemented.
In nonpatent reference 1, as IRAP pictures, IDR (Instantaneous Decoding Refresh) pictures, CRA (Clean Random Access) pictures and BLA (Broken Link Access) pictures are defined. At the time of random access from an IRAP picture, in the case of an IDR picture, normal decoding of a picture whose position in the decoding order is subsequent to that of the IDR picture is guaranteed, and in the case of a CRA picture and a BLA picture, normal decoding of a picture whose position in the decoding order and also in the display order are subsequent to that of any of the CRA picture and the BLA picture is guaranteed. In addition, a picture whose position in the display order is subsequent to that of an IRAP picture is surely subsequent to that of the IRAP picture in the decoding order so that pictures subsequent to the IRAP picture in the display order can be played back at the time of random access from the IRAP picture. More specifically, it is common that also for any IRAP picture, pictures subsequent to the IRAP picture in the display order can be normally decoded.
Further, also in nonpatent reference 2, the same random access function is provided. However, in nonpatent reference 2, only IDR pictures are defined as pictures for random access.